In injection molding of a thermoplastic resin, the resin is generally heated and melted at a temperature to obtain sufficient flowability of the resin for filling into a mold cavity. The flowability of the molten resin affects not only ease of filling into a mold cavity but also sufficiency of pressure transmission to the resin filled in the cavity. Therefore, it also affects dimensional precision and external appearance of molded articles, and transfer of fine information of the mold surface as required for molded articles such as optical disks. Thus, the molten resin flowability is an important factor for the moldability of a resin. An index of the flowability of the molten resin is the melt viscosity of a resin.
Thermoplastic resins have a high melt viscosity and is inferior in flowability as a molding material. This tends to cause poor appearance such as irregular gloss and weld-line of the molded article, poor transfer of fine pattern of a mold surface such as pits of an optical disk, and incomplete filling of a resin into a thin article portion in the mold, disadvantageously.
Conventionally, there are the following three methods for modifying resins to improve the flowability thereof. The first method is lowering of the molecular weight of a resin, for example, by lowering the average molecular weight or by broadening the molecular weight distribution to increase the content of low molecular components. This method is disadvantageous in that the impact strength and chemical resistance are deteriorated although the flowability is improved. The second method is introduction of a comonomer into the molecule. This method is disadvantageous in that the rigidity of the molded article is deteriorated at high temperatures. The third method is addition of a plasticizer such as a low molecular weight oily substance, e.g., a mineral oil, and a higher fatty acid ester. This method is disadvantageous in that the rigidity upon application of heat is deteriorated by the plasticizer, or the plasticizer adheres onto a mold during the molding operation to stain the mold.
With regard to molding conditions for increasing the flowability, it is effective to elevate a resin temperature or a mold temperature. However, higher temperatures of a resin cause thermal decomposition of the resin itself or of additives to thereby tend to deteriorate the strength of molded articles or to cause undesired foreign matter generation, mold staining, and resin discoloration due to the deterioration of the resin. Further, higher temperatures of a mold retard the cooling of a resin in a mold to lengthen the molding cycle time, disadvantageously.
On the other hand, it is known that when carbon dioxide is absorbed by a resin, it serves as a plasticizer to lower the glass transition temperature of the resin, as disclosed in many documents such as J. Appl. Polym. Sci., Vol.30, p. 2633 (1985). This phenomenon has not been widely utilized in resin molding. One of a few application examples thereof is a method disclosed in JP-A-5-318541 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") in which a gas such as carbon dioxide and nitrogen is dissolved into a thermoplastic resin, and the resin is filled into a cavity while the gas in the cavity is removed, to thereby improve the resin flowability and to produce molded articles without deterioration in strength and external appearance. In this method, however, the amount of the gas dissolved in the resin is as small as 0.18% by weight at the maximum if carbon dioxide is used as the gas. Such a small amount is insufficient to achieve the desired improvement of flowability. Since the cavity is kept at the atmospheric pressure or a reduced pressure in this method, the external appearance of the molded article surface is liable to be impaired by foaming caused at the flow front in the resin-filling step.
A counter pressure molding method is known as a technique of producing a foamed thick article with satisfactory appearance without surface sink or warpage using a resin containing a foaming agent, as disclosed in JP-B-62-16166 (The term "JP-B" as used herein means an "examined Japanese patent publication"). In this counter pressure molding, a molten resin containing a foaming gas is injected into a cavity filled with compressed air, and the compressed air in the cavity is then released out of the mold to cool the resin with maintaining the cavity pressure at a low pressure. In this method, foaming at the flow front is suppressed during the filling of the resin so that a molded article foamed only inside without foaming pattern on the surface of the molded article is produced. In the counter pressure molding, a cavity is almost fully filled with a molten resin of a non-foamed state, and thereby the molten resin inside a solidified surface layer formed during the filling of the resin is cooled and shrinks to cause foam formation at a degree corresponding to the volume shrinkage accompanied by the cooling. Therefore, it can be basically considered that the amount of the gas dissolved into the resin for imparting a foaming property to the resin be the minimum amount for compensating the volume shrinkage by foaming. Generally, the content of gas in a resin is less than 0.1% by weight with respect to nitrogen, and less than 0.15% by weight with respect to carbon dioxide. In the Example of JP-B-62-16166, the content of the nitrogen gas is estimated to be from 0.01 to 0.15% by weight, which cannot improve the resin flowability.
The object of the present invention is to provide an economical method for facilitating injection molding of a thermoplastic resin by reducing the viscosity of a molten resin without impairing the physical properties of the resin, the surface appearance of the molded article and productivity.
As a result of extensive studies to achieve the above object, the inventors of the present invention found that when a specific amount of carbon dioxide is dissolved in a molten resin, it serves as a plasticizer during only a molding process and diffuses out into the air after the molding. Thus, the viscosity of the molten resin can be reduced without changing the resin properties, to thereby facilitate the molding. The present invention has been accomplished by this finding.